The diffusion zone of sediment concentration in a small estuary and a coastal area with and without jetties was analyzed using numerical and physical models. It was assumed that sediment concentration was constant during intermediate flows, and was high in the beginning of high floods and decreased over time. The sediment diffusion zone and concentration for three cases, without the jetty, with only south jetty, and with the jetty, were examined in this study. In addition, the jetty was constructed in the field with the same design used in numerical and physical modeling and the diffusion zone of sediment concentration was monitored. The field data was similar to the results of numerical and physical modeling. Therefore, the reduction rate of sediment concentration by waves and the diffusion zone affected by increased sediment concentration were obviously decreased due to the construction of the jetty.